1. Field of the Invention
The present invention relates to silane coatings for metals. More particularly, the present invention provides silane coatings which not only provide improved adhesion to rubber and other polymers, but also provide corrosion protection (with or without a polymer layer).
2. Description of Related Art
Most metals are susceptible to corrosion, including the formation of various types of rust. Such corrosion will significantly affect the quality of such metals, as well as that of the products produced therefrom. Although rust and the like may often be removed, such steps are costly and may further diminish the strength of the metal. In addition, when polymer coatings such as paints, adhesives or rubbers are applied to the metals, corrosion may cause a loss of adhesion between the polymer coating and the metal.
By way of example, metallic coated steel sheet such as galvanized steel is used in many industries, including the automotive, construction and appliance industries. In most cases, the galvanized steel is painted or otherwise coated with a polymer layer to achieve a durable and aesthetically-pleasing product. Galvanized steel, particularly hot-dipped galvanized steel, however, often develops xe2x80x9cwhite rustxe2x80x9d during storage and shipment.
White rust (also called xe2x80x9cwet-storage stainxe2x80x9d) is typically caused by moisture condensation on the surface of galvanized steel which reacts with the zinc coating. On products such as GALVALUME(copyright), the wet-storage stain is black in color (xe2x80x9cblack rustxe2x80x9d). White rust (as well as black rust) is aesthetically unappealing and impairs the ability of the galvanized steel to be painted or otherwise coated with a polymer. Thus, prior to such coating, the surface of the galvanized steel must be pretreated in order to remove the white rust and prevent its reformation beneath the polymer layer. Various methods are currently employed to not only prevent the formation of white rust during shipment and storage, but also to prevent the formation of white rust beneath a polymer coating (e.g., paint).
In order to prevent white, rust on hot-dipped galvanized steel during storage and shipping, the surface of the steel is often passivated by forming a thin chromate film on the surface of the steel. While such chromate coatings do provide resistance to the formation of white rust, chromium is highly toxic and environmentally undesirable. It is also known to employ a phosphate conversion coating in conjunction with a chromate rinse in order to improve paint adherence and provide corrosion protection. It is believed that the chromate rinse covers the pores in the phosphate coating, thereby improving the corrosion resistance and adhesion performance. Once again, however, it is highly desirable to eliminate the use of chromate altogether. Unfortunately, however, the phosphate conversion coating is generally not very effective without the chromate rinse.
Recently, various techniques for eliminating the use of chromate have been proposed. In particular, various silane coatings have been developed for preventing corrosion of metal substrates. For example, U.S. Pat. No. 5,108,793 describes a technique of coating certain metal substrates with an inorganic silicate followed by treating the silicate coating with an organofunctional silane (U.S. Pat. No. 5,108,793). U.S. Pat. No. 5,292,549 teaches the rinsing of metallic coated steel sheet with a solution containing an organic silane and a crosslinking agent. Other silane coatings are described in U.S. Pat. Nos. 5,750,197 and 5,759,629, both of which are incorporated herein by way of reference.
Often, the corrosion protection provided by a particular silane coating will depend upon the identity of the metal substrate itself. In addition, the silane coating must also be compatible with any polymer layer to be applied over the silane coating (such as paints, adhesives or rubbers). For example, while a particular silane coating may provide excellent paint adhesion and corrosion protection, that same silane coating may provide little or no adhesion to certain rubbers. Thus, it is often necessary to tailor the silane coating to the specific application.
The silane coatings (or films) known heretofore are typically applied from an aqueous solution wherein the silane(s) are at least partially hydrolyzed. The resulting silane films, however, often contain residual water that can only be driven out by a high temperature heat treatment. Although the films are usually somewhat crosslinked, higher degrees of crosslinking typically require high temperature heat treatment (e.g., 200xc2x0 C.). These silane films are often very thin and fragile, and never completely pore-free or impervious to water. Therefore, corrosion may still occur to some extent when silane coated metals are exposed to a humid environment for a lengthy period of time. While high temperature heat treatment may help alleviate some of these problems, high temperature heat treatment may not always be practical. Thus, there is a need for a silane coating having improved mechanical properties and higher crosslink density, without the need for high temperature processing.
In addition to corrosion prevention, adhesive bonding between metals and rubber is also of interest. For example, many automobile components (such as tire cords and vibration dampers) rely on adhesive bonding between a metal substrate and a sulfur-cured rubber. Steel tire cords, for example, are typically coated with a thin layer of brass in order to promote adhesion between the underlying steel and the sulfur-cured rubber. In addition, adhesion promoters such as cobalt salt additives, and HRH systems (hexamethylene tetramine, resorcinol and hydrated silica) are also used to further enhance rubber adhesion for tire cords. Solvent-based adhesive systems are used in other applications for bonding metals to sulfur-cured rubbers. Although the performance of the various methods currently employed is adequate, they still suffer from several drawbacks. Cobalt salts, for example, are expensive and pose availability problems, while brass stimulates galvanic corrosion in conjunction with steel. Solvent-based adhesives are flammable and hence hazardous.
Although certain silanes have been found to promote adhesion between a metal substrate and a polymer layer, the results are typically system dependent. In other words, the amount of adhesion provided by a particular silane coating typically depends on the metal substrate as well as the polymer layer to be adhered thereto. For example, while certain silane solutions may provide improved adhesion between a metal substrate and a peroxide-cured rubber, these same silane solutions will often not provide the same results for sulfur-cured rubber. Thus, there is also a need for methods of improving the adhesion between a metal substrate and a polymer layer, particularly sulfur-cured rubber.
It is an object of the present invention to provide silane coatings on a metal substrates for improving corrosion resistance and/or polymer adhesion.
It is another object of the present invention to provide silane coatings which provide improved adhesion to rubber, including sulfur-cured and peroxide-cured rubber.
The foregoing objects, in accordance with one aspect of the present invention, are provided by a method of treating a metal substrate, comprising:
(a) providing a metal substrate; and
(b) applying a coating of a silane composition onto the metal substrate, the silane composition comprising at least one substantially unhydrolyzed aminosilane which has one or more secondary or tertiary amino groups.
Suitable aminosilanes include: 
wherien:
n is either 1 or 2;
y=(2xe2x88x92n);
each R1 is individually chosen from the group consisting of: C1-C24 alkyl and C2-C24 acyl;
each R2 is individually chosen from the group consisting of: substituted aliphatic groups, unsubstituted aliphatic groups, substituted aromatic groups, and unsubstituted aromatic groups;
R5 is chosen from the group consisting of: hydrogen, C1-C10 alkyl, C1-C10 alkyl substituted with one or more amino groups, C1-C10 alkenyl, C1-C10 alkenyl substituted with one or more amino groups, aryl, and alkylaryl;
X is either: 
wherein each R3 is individually chosen from the group consisting of: hydrogen, substituted and unsubstituted aliphatic groups, and substituted and unsubstituted aromatic groups; and
R4 is chosen from the group consisting of: substituted and unsubstituted aliphatic groups, and substituted and unsubstituted aromatic groups; and
wherein, when n=1, at least one of the R3 and the R5 is not hydrogen (else the aminosilane would contain no secondary or tertiary amino group).
Particularly preferred aminosilanes include bis-silyl aminosilanes having two trisubstituted silyl groups, wherein the substituents are individually chosen from the group consisting of alkoxy, aryloxy and acyloxy. Suitable bis-silyl aminosilanes include: 
wherein:
each R1 is individually chosen from the group consisting of: C1-C24 alkyl and C2-C24 acyl;
each R2 is individually chosen from the group consisting of: substituted aliphatic groups, unsubstituted aliphatic groups, substituted aromatic groups, and unsubstituted aromatic groups; and
X is either: 
wherein each R3 is individually chosen from the group consisting of: hydrogen, substituted and unsubstituted aliphatic groups, and substituted and unsubstituted aromatic groups; and
R4 is chosen from the group consisting of: substituted and unsubstituted aliphatic groups, and substituted and unsubstituted aromatic groups.
Exemplary bis-silyl aminosilanes include: bis-(trimethoxysilylpropyl)amine, bis-(triethoxysilylpropyl)amine, bis-(triethoxysilylpropyl)ethylene diamine, N-[2-(vinylbenzylamino)ethyl]-3-aminopropyltrimethoxy silane, and aminoethyl-aminopropyltrimethoxy silane.
Although the coating provided by the unhydrolyzed aminosilane composition provides improved corrosion protection, the composition may further include at least one xe2x80x9cotherxe2x80x9d substantially unhydrolyzed silane (i.e., an unhydrolyzed silane other than an aminosilane having at least one secondary or tertiary amino group). In particular, organofunctional silanes such as at least one substantially unhydrolyzed bis-silyl polysulfur silane, may be included in order to provide improved adhesion to a polymer (such as a paint, an adhesive, or a rubber, including sulfur-cured rubber). Suitable xe2x80x9cotherxe2x80x9d silanes include bis-silyl polysulfur silanes comprising: 
wherein each R1 is an alkyl or an acetyl group, and Z is xe2x80x94Qxe2x80x94Sxxe2x80x94Qxe2x80x94, wherein each Q is an aliphatic or aromatic group, and x is an integer of from 2 to 10. An exemplary bis-silyl polysulfur silane is bis-(triethoxysilylpropyl) tetrasulfide.
In one preferred embodiment, one or more substantially unhydrolyzed bis-silyl aminosilanes are combined with one or more substantially unhydrolyzed bis-silyl polysulfur silanes to provide a silane composition which may be applied to a metal substrate. The resultant silane coating not only provides corrosion protection (even without a polymer coating thereover), but also provides surprisingly improved adhesion to polymers such as paints, adhesives, and rubbers. In particular, the silane coating provides improved adhesion to sulfur-cured rubbers, as well as peroxide-cured rubbers. Uncured (or even cured) rubber compounds are simply applied directly on the silane coating, and then cured in the usual fashion (or, if already cured, the rubber is simply heated while applying pressure). Sulfur-cured and peroxide-cured rubber compounds known to those skilled in the art may be adhered to metal substrates in this manner, using standard rubber curing methods also known to those skilled in the art.
The above mixture of one or more bis-silyl aminosilanes and one ore more bis-silyl polysulfur silanes may also be provided as a partially or substantially hydrolyzed silane solution. This hydrolyzed silane solution also provides surprising adhesion to polymers, particularly sulfur-cured and peroxide-cured rubbers.
In U.S. Pat. No. 5,750,197, it was demonstrated that an aqueous solution of a hydrolyzed bis-functional silane (such as 1,2-bis-(trimethoxysilylpropyl) amine) provides corrosion protection. Applicants have now found that mixtures of at least one aminosilane and at least one bis-silyl polysulfur silane provide further improvements in both corrosion protection and polymer adhesion (particularly to sulfur-cured rubber). The silanes do not have to be hydrolyzed, as previously believed, since these silane mixtures provide corrosion protection and enhanced polymer adhesion whether hydrolyzed or unhydrolyzed. In addition, one or more unhydrolyzed aminosilanes may even be applied to a metal substrate alone (with or without the addition of other silanes), since applicants have found that silane coatings applied in this manner will form a dry film which provides corrosion protection. It is believed that secondary or tertiary amino groups in the aminosilane will crosslink, even if the aminosilane has not been hydrolyzed, thereby allowing the formation of a dry film from a one or more unhydrolyzed silanes.
The solutions and methods of the present invention may be used on a variety of metals, including (but not limited to):
zinc and zinc alloys, such as titanium-zinc (zinc which has a very small amount of titanium added thereto), zinc-nickel alloy (typically about 5% to about 13% nickel content), and zinc-cobalt alloy (typically about 1% cobalt);
metal substrates having a zinc-containing coating, such as galvanized steel (especially hot dipped galvanized steel (xe2x80x9cHDGxe2x80x9d) and electrogalvanized steel (xe2x80x9cEGSxe2x80x9d)), GALVALUME(copyright) (a 55%-Al/43.4%-Zn/1.6%-Si alloy coated sheet steel manufactured and sold, for example, by Bethlehem Steel Corp), GALFAN(copyright) (a 5%-Al/95%-Zn alloy coated sheet steel manufactured and sold by Weirton Steel Corp., of Weirton, W.V.), galvanneal (annealed hot dipped galvanized steel) and similar types of coated metals;
steel, particularly cold rolled steel and carbon steel;
aluminum and aluminum alloys;
copper and copper alloys, such as brass; and
tin and tin alloys, including metal substrates (such as CRS) having tin-containing coatings
The silane solutions and mixtures of the present invention may be applied to the metal prior to shipment to the end-user, and provide corrosion protection during shipment and storage. The coated metal may be used as is, or, more preferably the end user may apply a polymer layer (such as paint, rubber, or adhesive) directly on top of the silane coating in the usual manner. The silane coatings of the present invention not only provide excellent corrosion protection, but also provide superior adhesion between the metal substrate and the polymer layer. These silane coatings (or films) are also very durable and highly deformable, and therefore provide significant corrosion protection even after deep drawing of the coated metal. The methods and compositions of the present invention are particularly useful for bonding metals to rubber, including sulfur-cured rubber. In fact, the present invention provides improved adhesion between metals and sulfur-cured rubber without the need for cobalt adhesion promoters in the rubber composition.
The silane compositions of the present invention comprise one or more aminosilanes and one or more bis-silyl polysulfur silanes. The solutions do not require the use or addition of silicates or aluminates, and eliminate the need for phosphate conversion coatings. The silanes in the treatment solution may be substantially unhydrolyzed, partially hydrolyzed, or substantially fully hydrolyzed. As used herein, the term xe2x80x9csubstantially unhydrolyzedxe2x80x9d simply means that the silane(s) are applied either in a pure state (no added solvents) or from a solution which does not include water. However, it is recognized that such silane(s) may absorb some water from the atmosphere, hence the term xe2x80x9csubstantially unhydrolyzedxe2x80x9d (water is not purposefully added to the silane(s)).
The silane solutions of the present invention may also comprise one or more substantially unhydrolyzed aminosilanes having at least one secondary or tertiary amino group, with or without additional silanes or non-aqueous solvents. Applicants have found that these unhydrolyzed aminosilanes will dry to a hard film at room temperature (typically in 30 minutes or less. The silane coating applied from one or more unhydrolyzed aminosilanes will readily crosslink, and will even crosslink other silanes which are included in the coating. This finding is quite surprising, since conventional wisdom suggests that silanes should only be applied to metals in a hydrolyzed state (i.e., from a solution which includes water).
Hydrolyzed Silane Solutions
The hydrolyzed silane solutions of the present invention preferably comprise one or more bis-silyl aminosilanes and one or more bis-silyl polysulfur silanes. These hydrolyzed silane solutions also preferably include water (for hydrolysis), and one or more compatible solvents (such as ethanol, methanol, propanol or isopropanol) in order to solubilize the polysulfur silane. The amount of solvent employed will depend upon the concentration of polysulfur silane(s) in the solution, and the solution should therefore include sufficient solvent to solubilize the polysulfur silane(s). The ratio of water to solvent in the silane solution (as applied to the metal substrate, by volume) may be between about 1:99 and about 99:1, more preferably between about 1:1 and about 1:20.
It is preferred (but not required) that the two silanes (bis-silyl aminosilane and bis-silyl polysulfur silane) are hydrolyzed separately before being mixed with one another to form the silane solution which is applied to the metal substrate.
Thus, one or more bis-silyl aminosilanes may be hydrolyzed by mixing the silane(s) with water at the desired concentration. A compatible solvent (such as an alcohol) may be added as desired, however hydrolysis of the bis-silyl aminosilane will generally proceed to completion without an added solvent (and the resulting hydrolyzed silane solution will remain stable). One or more bis-silyl polysulfur silanes may be hydrolyzed in a similar fashion, however a compatible solvent should be included since these silanes are generally not water-soluble. Thus, hydrolysis of the bis-silyl polysulfur silane(s) may take place in a solution having a water to solvent ratio (by volume) of between about 1:99 and about 99:1, more preferably between about 1:1 and about 1:20.
As an alternative to employing an organic solvent, the hydrolyzed silanes (particularly the polysulfur silane(s)) may be prepared as an emulsion without a solvent. The silane(s) is simply mixed with water and a suitable surfactant known to those skilled in the art. An emulsified, hydrolyzed solution of a bis-silyl polysulfur silane can be prepared, for example, by mixing a 5% solution of the silane in water along with 0.2% of a surfactant (by volume). Suitable surfactants include, for example, sorbitan fatty acid esters (such as Span 20, available from ICI Surfactants). Once the emulsion of polysulfur silane has been prepared, it may simply be mixed with one or more hydrolyzed bis-silyl aminosilanes as described below and then applied to the metal substrate.
During hydrolysis, the xe2x80x94OR1 groups in the bis-silyl aminosilane(s) and the bis-silyl polysulfur silane(s) (as defined below) are replaced by hydroxyl groups. In order to accelerate silane hydrolysis and avoid silane condensation during hydrolysis, the pH may be maintained below about 10, more preferably between about 4 and about 9 (particularly for hydrolysis of the bis-silyl aminosilane). The pH ranges preferred during solution preparation should not be confused with the application pH (i.e., the pH of the silane solution applied to the metal substrate). The pH may be adjusted, for example, by the addition of one or more compatible acids, preferably organic acids such as acetic, formic, propionic or iso-propionic. Sodium hydroxide (or other compatible base) may be used, if needed, to raise the pH of the silane solution. Some silanes provide an acidic pH when mixed with water alone, and for these silanes pH adjustment may not be needed to accelerate silane hydrolysis. The individual silane solutions are preferably stirred for at least 24 hours to ensure complete hydrolysis. In the case of the solution of bis-silyl polysulfur silane(s), hydrolysis may be allowed to proceed for several days (such as 3-4 days, or more) for optimal performance. Once the individual silane solutions have been separately hydrolyzed, they are mixed with one another prior to application on the metal substrate. The hydrolyzed silane mixtures, however, are stable for at least up to 30 days, and therefore need not be used immediately after mixing.
It should be noted that the various silane concentrations discussed and claimed herein are all defined in terms of the ratio between the amount (by volume) of unhydrolyzed silane(s) employed to prepare the treatment solution (i.e., prior to hydrolyzation), and the total volume of treatment solution components (i.e., aminosilanes, polysulfur silanes, water, optional solvents and optional pH adjusting agents). In the case of aminosilane(s), the concentrations herein (unless otherwise specified) refer to the total amount of unhydrolyzed aminosilanes employed, since multiple aminosilanes may optionally be present. The polysulfur silane(s) concentrations herein are defined in the same manner. During preparation of the individual hydrolyzed silane solutions, the silane concentration in each may vary significantly from the desired total silane concentration in the mixed silane solution (i.e., the solution which is applied to the metal substrate). It is preferred, however, that the silane concentration in the individual hydrolyzed solutions is approximately the same as the desired total silane concentration in the mixed silane solution in order to simplify the final mixing step. In this manner, the final mixed silane solution may be prepared simply by mixing the appropriate ratio of the individual silane solutions.
As for the concentration of hydrolyzed silanes in the final, mixed silane solution (i.e., the solution applied to the metal substrate) beneficial results will be obtained over a wide range of silane concentrations and ratios. It is preferred, however, that the hydrolyzed solution have at least about 0.1% silanes by volume, wherein this concentration refers to the total concentration of bis-silyl aminosilane(s) and bis-silyl polysulfur silane(s) in the solution. More preferably, the solution has between about 0.1% and about 10% silanes by volume. As for the ratio of bis-silyl aminosilane(s) to bis-silyl polysulfur silane(s) in the hydrolyzed silane solution, a wide range of silane ratios provide beneficial results. Preferably, however the ratio of bis-silyl aminosilane(s) to bis-silyl polysulfur silane(s) is between about 1:99 and about 99:1. More preferably, particularly when the hydrolyzed silane solution is to be used for rubber bonding, this ratio is between about 1:10 and about 10:1. Even more preferably, this ratio is between about 1:3 and about 3:1.
The term xe2x80x9capplication pHxe2x80x9d refers to the pH of the silane solution when it is applied to the metal surface, and may be the same as, or different from the pH during solution preparation. When used to improve the adhesion of a rubber (particularly sulfur-cured rubber) to a metal, the application pH is preferably between about 4 and about 7, most preferably between about 4 and about 5. The pH of the mixed silane solution may be adjusted in the manner described previously.
The metal surface to be coated with the mixed hydrolyzed silane solution of the present invention may be solvent and/or alkaline cleaned by techniques well-known to those skilled in the art prior to application of the silane solution. The hydrolyzed silane solution (prepared in the manner described above) is applied to the metal surface (i.e., the sheet is coated with the silane solution) by, for example, dipping the metal into the solution (also referred to as rinsing), spraying the solution onto the surface of the metal, or even brushing or wiping the solution onto the metal surface. Various other application techniques well-known to those skilled in the art may also be used. When the preferred application method of dipping is employed, the duration of dipping is not critical, as it generally does not significantly affect the resulting film thickness. It is merely preferred that whatever application method is used, the contact time should be sufficient to ensure complete coating of the metal (such as 10 seconds or more).
After coating with the silane solution of the present invention, the metal substrate may simply be air-dried at room temperature. Heated drying is not preferred if the hydrolyzed silane coating is to be used for improving rubber adhesion, since it is preferred that the coating remain only partially crosslinked. While heated drying (or curing) promotes crosslinking, too much crosslinking in the silane coating may prevent sufficient adhesion between a rubber and the metal substrate. Of course the amount of crosslinking can be tailored to suit one""s particular needs (such as the desired bond strength between the metal substrate and rubber), and the present invention is therefore not limited to silane coatings dried only at room temperature. Once dried, the treated metal may be shipped to an end-user, or even stored for later use.
The coatings applied from hydrolyzed silane solutions of the present invention provide significant corrosion resistance during both shipping and storage. More importantly, this silane coating need not be removed prior to application of a polymer layer on top of the silane coating. Thus, the end-user, such as an automotive manufacturer, may apply a polymer (such as a paint, an adhesive or rubber) directly on top of the silane coating without additional treatment (such as the application of chromates or the use of solvent-based adhesives). The hydrolyzed silane coatings of the present invention not only provide a surprisingly high degree of adhesion to the polymer layer, but also prevent delamination and underpaint corrosion even if a portion of the base metal is exposed to the atmosphere.
As reported in application Ser. No. 09/356,927, now abandoned, titled Silane Treatment for Electrocoated Metals, which names Wim J. van Ooij and Guru P. Sundararajan as inventors, was filed on even date herewith, and the disclosure of which is incorporated herein by way of reference, the hydrolyzed silane solutions of the present invention having both a bis-silyl aminosilane and a bis-silyl polysulfur silane also provide excellent adhesion to paints (particularly electrocoats) and other polymeric adhesives. As also reported therein, and as incorporated herein by way of reference, the hydrolyzed silane solutions of the present application also provide improved corrosion protection, as well as adhesion to other non-rubber polymers (such as paints and adhesives).
The hydrolyzed silane coatings of the present invention are particularly useful for bonding rubber to various metal substrates, particularly sulfur-cured rubbers such as natural rubber (xe2x80x9cNRxe2x80x9d), NBR, SBR and EPDM compounds. The uncured rubber compound is merely applied directly on top of the silane coating, and is then cured in the typical fashion (i.e., using the cure conditions required for the particular rubber compound employed). Even previously cured rubber may be adhered to the metals using the silane coatings of the present invention simply by applying the cured rubber compound directly on top of the silane coating and thereafter applying sufficient heat and pressure to adhere the rubber to silane coating (and hence to the metal substrate). Thus, the silane coatings provided by the hydrolyzed silane solutions of the present invention provide improved rubber adhesion, as well as significant corrosion protection. These results are surprising since a coating applied from a solution of a hydrolyzed bis-silyl aminosilane alone provides no adhesion to sulfur-cured rubber. Yet, when a solution comprising the same bis-silyl aminosilane and a hydrolyzed bis-silyl polysulfur silane is used, the resulting adhesion exceeds that provided by the bis-silyl polysulfur silane alone. Previously reported silane coatings which provide improved adhesion to peroxide-cured rubbers, likewise provide no adhesion to sulfur-cured rubbers. An added benefit of the improved adhesion provided by the hydrolyzed silane coatings of the present invention is that the sulfur-cured rubbers may be formulated without cobalt adhesion promoters, since the silane coatings of the present invention improve rubber adhesion without such promoters.
Since the hydrolyzed silane coatings of the present invention also provide corrosion protection and bonding to polymers other than rubber (such as paint), the present invention has the added benefit of providing silane solutions and methods which may be used in a myriad of applications. Thus, manufacturers need not use one silane solution for corrosion protection, another for paint adhesion, yet another for adhesion to peroxide-cured rubber, and still another for adhesion to sulfur-cured rubber. The hydrolyzed silane solutions of the present invention are suitable for all of these applications on a variety of metals.
The preferred bis-silyl aminosilanes which may be employed in the present invention have two trisubstituted silyl groups, wherein the substituents are individually chosen from the group consisting of alkoxy, aryloxy and acyloxy. Thus, these bis-silyl aminosilanes have the general structure: 
wherein each R1 is chosen from the group consisting of: C1-C24 alkyl (preferably C1-C6 alkyl), and C2-C24 acyl (preferably C2-C4 acyl). Each R1 may be the same or different, however, in the hydrolyzed silane solutions of the present invention, at least a portion (and preferably all or substantially all) of the R1 groups are replaced by a hydrogen atom. Preferably, each R1 is individually chosen from the group consisting of: ethyl, methyl, propyl, iso-propyl, butyl, iso-butyl, sec-butyl, ter-butyl and acetyl.
Each R2 in the aminosilane(s) may be a substituted or unsubstituted aliphatic group, or a substituted or unsubstituted aromatic group, and each R2 may be the same or different. Preferably, each R2 is chosen from the group consisting of: C1-C10 alkylene, C1-C10 alkenylene, arylene, and alkylarylene. More preferably, each R2 is a C1-C10 alkylene (particularly propylene).
X may be: 
wherein each R3 may be a hydrogen, a substituted or unsubstituted aliphatic group, or a substituted or unsubstituted aromatic group, and each R3 may be the same or different. Preferably, each R3 is chosen from the group consisting of hydrogen, C1-C6 alkyl and C1-C6 alkenyl. More preferably, each R3 is a hydrogen atom.
Finally, R4 in the aminosilane(s) may be a substituted or unsubstituted aliphatic group, or a substituted or unsubstituted aromatic group. Preferably, R4 is chosen from the group consisting of: C1-C10 alkylene, C1-C10 alkenylene, arylene, and alkylarylene. More preferably, R4 is a C1-C10 alkylene (particularly ethylene).
Exemplary preferred bis-silyl aminosilanes which may be used in the present invention include:
bis-(trimethoxysilylpropyl)amine (which is sold under the tradename A-1170 by Witco): 
bis-(triethoxysilylpropyl)amine: 
and bis-(triethoxysilylpropyl)ethylene diamine: 
The preferred bis-silyl polysulfur silanes which may be employed in the present invention include: 
wherein each R1 is as described before. In the hydrolyzed silane solutions of the present invention, at least a portion (and preferably all or substantially all) of the R1 groups are replaced by a hydrogen atom. Z is xe2x80x94Qxe2x80x94Sxxe2x80x94Qxe2x80x94, wherein each Q is an aliphatic (saturated or unsaturated) or aromatic group, and x is an integer of from 2 to 10. Q within the bis-functional polysulfur silane can be the same or different. In a preferred embodiment, each Q is individually chosen from the group consisting of: C1-C6 alkyl (linear or branched), C1-C6 alkenyl (linear or branched), C1-C6 alkyl substituted with one or more amino groups, C1-C6 alkenyl substituted with one or more amino groups, benzyl, and benzyl substituted with C1-C6 alkyl.
Particularly preferred bis-silyl polysulfur silanes include bis-(triethoxysilylpropyl) sulfides having 2 to 10 sulfur atoms. Such compounds have the following formula: 
wherein x is an integer of from 2 to 10. One particularly preferred compound is bis-(triethoxysilylpropyl) tetrasulfide (also referred to as bis-(triethoxysilylpropyl) sulfane, or xe2x80x9cTESPTxe2x80x9d). Commercially-available forms of TESPT (such as A-1289, available from Witco), however, are actually mixtures of bis-(triethoxysilylpropyl) sulfides having 2 to 10 sulfur atoms. In other words, these commercially-available forms of TESPT have a distribution of sulfide chain lengths, with the S3 and S4 sulfides predominating. Thus, the scope of the present invention includes hydrolyzed silane solutions containing mixtures of bis-silyl polysulfur silanes in combination with one or more bis-silyl aminosilanes.
Coatings Applied from Unhydrolyzed Silanes
Applicants have also surprisingly found that unhydrolyzed silanes may be applied directly onto the metal substrate in order to not only provide corrosion protection, but also to provide improved adhesion to polymer layers (such as paints, adhesives, or rubbers). Preferably, at least one aminosilane having at least one secondary or tertiary amino group (such as the bis-silyl aminosilanes described above) is applied to the metal substrate in a substantially unhydrolyzed state (i.e., the R1 groups are not replaced by a hydrogen atom). One or more additional silanes (organofunctional or non-organofunctional) may also be mixed with the aminosilane, however even an unhydrolyzed aminosilane by itself provides corrosion protection.
The aminosilanes which may be applied to a metal substrate in an unhydrolyzed state include aminosilanes having at least one secondary or tertiary amino group. Suitable unhydrolyzed aminosilanes include: 
wherein n is either 1 or 2. Thus, when a bis-silyl aminosilane of the type described previously is employed, n=2. Each R1 is as defined previously, each R2 is as described previously, and X is as described previously. R5 may be hydrogen, C1-C10 alkyl, C1-C10 alkyl substituted with one or more amino groups, C1-C10 alkenyl, C1-C10 alkenyl substituted with one or more amino groups, aryl, and alkylaryl. When n=1, R3 and R5 should not all be hydrogen (else the aminosilane will have no secondary or tertiary aminosilane groups). Particularly preferred aminosilanes include the bis-silyl aminosilanes (i.e., n=2, y=0) described previously, as well as diamino silanes. Suitable diaminosilanes include those silanes having at least one trialkoxysilyl or triacetoxysilyl group, as well as two amino groups, at least one of which is a secondary amino group. Suitable diamino silanes include N-[2-(vinylbenzylamino)ethyl]-3-aminopropyltrimethoxy (xe2x80x9cSAAPSxe2x80x9d), aminoethyl-aminopropyltrimethoxy silane (xe2x80x9cAEPSxe2x80x9d), and bis-(triethoxysilylpropyl)ethylene diamine (a bis-silyl, diamino silane).
The unhydrolyzed secondary or tertiary aminosilane(s) may also be combined with one or more additional unhydrolyzed silanes, particularly organofunctional silanes, prior to application to the metal substrate. In this fashion, one can tailor the silane coating to the particular application. While one or more unhydrolyzed secondary or tertiary aminosilane(s) will provide a durable, corrosion-preventing film, applicants have found that the addition of one or more other unhydrolyzed silanes (particularly organofunctional silanes, such as bis-silyl polysulfur silanes), will not only provide corrosion resistance but also polymer adhesion. xe2x80x9cOrganofunctional silanexe2x80x9d simply means a silane having one or more trisubstituted silyl groups, and one or more organofunctional groups.
One preferred embodiment of the present invention comprises a mixture of one or more aminosilanes having at least one secondary or tertiary amino group (especially the bis-silyl aminosilanes described previously) and one or more bis-silyl polysulfur silanes (as described above), with the silanes in a substantially unhydrolyzed state. The silane(s) (whether an aminosilane by itself, or a mixture of one or more aminosilanes and one or more other silanes) may be applied to the metal substrate as a pure silane mixture (i.e., no solvents or water), or may be diluted with a compatible solvent (other than water). Suitable solvents, for example, include, ethanol, methanol, propanol and isopropanol. Diluting the unhydrolyzed silane(s) with a compatible solvent allows the thickness of the silane to be controlled.
The unhydrolyzed silane(s) are simply coated onto the metal substrate, such as by wiping, dipping or spraying the silane (or silane mixture) onto the metal. Thereafter, the silane coating is dried. If the unhydrolyzed silane mixture applied to the metal substrate contains only aminosilanes, the coating is preferably dried at room temperature. Alternatively, the aminosilane coating may be dried by heating to a temperature of between about 100xc2x0 and about 25xc2x0 for a period of time sufficient to form a dry film (i.e., silane coating).
When the unhydrolyzed silane mixture applied to the metal substrate includes both an aminosilane (such as a bis-silyl aminosilane) and an organofunctional silane (such as a bis-silyl polysulfur silane), the silane coating is preferably dried by heating the coated metal to a temperature of between about 100xc2x0 C. and about 250xc2x0 C. (more preferably to a temperature of between about 100xc2x0 C. and about 160xc2x0 C., for a period of time sufficient to form a dry film (such as about 10 to about 60 minutes). It should be noted, however, that the amount of drying time will obviously vary with the drying temperature, as well as the nature of the silane coating (e.g., silane concentrations, amount of organic solvent, etc.). Thus, longer periods of time than that specified may be used. As the coating dries, the unhydrolyzed silanes will become partially crosslinked, thereby forming a semi-crosslinked silane coating which not only provides significant corrosion protection, but also improved adhesion to polymers such as paints, adhesives and rubbers (particularly sulfur-cured rubbers). When the unhydrolyzed silane coating is used for polymer adhesion (particularly to sulfur-cured rubber), the silane coated metal should merely be dried for a period of time sufficient to form a dry film (i.e., so that the silane film is semi-crosslinked). When the silane film is intended to provide corrosion protection only, the silane film may be fully crosslinked (or cured) simply by heating the silane coated metal substrate for a longer period of time and/or at a higher temperature. While a fully crosslinked silane film is not desired for polymer adhesion, it will provide significant corrosion protection.
The unhydrolyzed silane mixtures of the present invention are prepared simply by mixing the silanes with one another in their pure, unhydrolyzed state in the desired ratio. The resulting silane mixture may be diluted with a compatible solvent, if desired. If a solvent is used, the total silane concentration should be at least about 10% (by volume), more preferably at least about 25%.
As for the ratio of unhydrolyzed aminosilane(s) having at least one secondary or tertiary amino group (such as unhydrolyzed bis-silyl aminosilane(s)) to other unhydrolyzed silane(s) (such as unhydrolyzed bis-silyl polysulfur silane(s)), a wide range of silane ratios provide beneficial results. Preferably, however the ratio of aminosilane(s) to other silanes is between about 1:10 and about 10:1. More preferably, particularly when the unhydrolyzed silane mixture is to be used for rubber bonding, this ratio is between about 1:3 and about 1:1.
Applicants have found that unhydrolyzed silane compositions-having at least one secondary or tertiary aminosilane (particularly at least one bis-silyl aminosilane) provide a silane coating which is highly resistant to corrosion. When the silane compositions include one or more organofunctional silanes (particularly at least one bis-silyl polysulfur silane), the resultant silane coating provides superior adhesion to polymers (such as paints, adhesives and rubbers). In fact, the adhesion to sulfur-cured rubbers is even greater than that provided by the hydrolyzed silane solutions of the present invention. Like the hydrolyzed silane solutions discussed previously, the unhydrolyzed silane compositions of the present invention provide unexpectedly high levels of adhesion to a variety of rubber compositions, including sulfur-cured rubber such as high-sulfur rubber systems, low-sulfur rubber systems, EV rubber systems, and semi-EV rubber systems.